This invention relates broadly to the field of abradable coatings and particularly to a material which is flame sprayed onto a substrate to produce an abradable coating thereon.
Flame spraying involves heat softening of a heat fusable material, such as a metal or a ceramic, and propelling the softened or molten material in fine particulate form against the surface to be coated. The heat softened or melted material, on striking the surface, becomes bonded thereto.
Typical flame spray guns use either a combustion or a plasma flame to provide the heat for melting the powder, although other heating means, such as electric arcs, resistance heaters or induction heaters may be used alone or in combination with a flame spray gun. In a powder-type combustion flame spray gun, the carrier gas for the powder can be one of the combustion gases, or it can be compressed air. In a plasma flame spray gun, on the other hand, the primary plasma gas is generally nitrogen or argon. Hydrogen or helium is usually added to the primary gas. The carrier gas is generally the same as the primary plasma gas, although other gases, such as hydrocarbons, are used in certain situations.
The nature of a coating obtained by flame spraying a metal or ceramic powder can be quite specifically controlled by proper selection of the composition of the powder, control of the physical nature of the powder and use of select flame spraying conditions. For example, it is well known and common practice to flame spray a simple mixture of ceramic powder and metal powder. Coatings produced by spraying mixtures usually contain both the ceramic and the metal material that has been flame sprayed and have desirable characteristics such as being abradable, hard, erosion resistant etc., depending on the materials being sprayed and the spraying conditions.
Abradable thermal barrier coatings require a highly porous coating network of 20-35% porosity, which cannot be achieved by conventional flame spray techniques. The porosity levels achieved by such conventional techniques for ceramic coatings using conventional powders normally range between 5 and 20%, and the porosity level, it has been found, is a direct function of the powder size and spraying parameters, e.g., spray rate, spray distance and power levels of the spray gun.
Another approach for producing an abradable coating is described in U.S. Pat. No. 4,299,865 wherein an abradable material is codeposited on the substrate to be coated with a thermally decomposable filler powder. Once the desired coating thickness is achieved, the coated substrate is heated to a temperature high enough to decompose the filler powder thereby leaving an abradable coating, which is about 20 to 30% void.
This approach requires that the coated article be subjected to heat in order to decompose the filler powder. This may be inconvenient or difficult depending on the physical size of the coated article. Additionally, the process described is likely to require very accurate control in order to reliably produce the desired coating.
Because abradable coatings are highly desirable in certain applications, such as clearance control in gas turbine engines, the problem of developing an abradable coating using flame spraying techniques has been investigated by others in order to obtain the desired levels of porosity. In addition to the above approach, yet another appoach has been investigated. This approach utilizes a temperature-resistant aluminum silicate hollow sphere filler (e.g., Eccospheres.TM.) which is ultimately distributed throughout the ceramic coating and remains intact, even after exposure to elevated temperatures.
There are several problems with Eccosphere.TM. sprays. One problem is that the material does not spray well, i.e., the amount of material which can be sprayed in a given time period is small. Coatings so produced also have limited cohesive bond strength and are very friable. The material additionally has a low melting point so it is not particularly suitable for use in high temperature environments.
Accordingly, it is the principal objective of the present invention to provide a powder for flame spraying onto a substrate a coating which is abradable.
It is still another objective of this invention to provide a flame spray powder for producing an abradable coating which is not expensive to produce.
It is still a further objective of the invention to provide a powder for producing an abradable coating which is suitable for use on parts which are used at high temperatures.